Researchers have more questions than answers about the latest bird flu circulating in China, including whether birds are the only reservoir for the virus.

The World Health Organization (WHO) reports that 17 people in China have died of H7N9 infection, and there is no evidence of person-to-person transmission of the flu virus. However, while experts assume that those infected were exposed when they came in contact with sick birds, most likely poultry, Chinese health officials said that 40% of the 82 people who had fallen ill so far had not reported any contact with live poultry.

Why? Joseph Bresee, chief of the epidemiology-and-prevention branch in the influenza division of the U.S. Centers for Disease Control and Prevention (CDC), says that percentage may change, as more information becomes available and patients recall even casual contact they might have had with poultry. A more worrisome possibility, expressed by some experts in China, who believe a young boy may have infected his brother, is that H7N9 may actually be passing from person to person. So far, there is no evidence of what experts call sustained human transmission, but investigators are looking into how the virus emerged in the family members.

Influenza experts are also considering the chance that H7N9, a new influenza virus that has never been seen in people before, may have another, non-avian-animal home.

That possibility is bolstered by the fact that unlike previous H7 viruses, which are primarily found in birds, H7N9 appears to be adept at infecting mammals. “This virus is different from other H7 viruses isolated before from birds,” says Bresee. “There are some mutations in this virus that seem to make it better adapted to infecting mammalian hosts compared to normal avian viruses.”

Once it does infect a human host, it also causes relatively severe disease, which is unusual for an avian-flu strain. That’s worrisome since most people will not have built-in immunity against bird-flu strains since we aren’t likely to be infected with them. “It’s not a virus that the human population has seen much of,” says Bresee. “So we expect that the whole population of humans is susceptible to infection with this virus. That’s concerning.”

Based on the genetic analysis of the strains isolated from infected patients so far, however, there are no hints that H7N9 has mutated to pick up more human-flu-virus characteristics. Often, if people are infected with influenza and come in contact with an animal strain, the strains will reassort to generate a potentially more virulent form of flu.

That leaves the possibility that H7N9 is replicating in animal hosts, such as cats or pigs, other than birds. While influenza strains from different animal hosts tend to bear specific genetic signatures that make it easier for them to infect different animals, Bresee says it’s possible that a bird flu could pass through different animal species without changing considerably and then reappear in poultry again. “These viruses are promiscuous and able to mutate, and it’s not surprising to find the virus that we are looking at now in humans appear in another mammal like a cat or pig,” he says.

For now, it’s too early to draw any conclusions about other animal reservoirs of H7N9, but the U.S. CDC is working closely with Chinese health officials to better understand how H7N9 jumped from birds to people and how potentially virulent it could become. So far, H7N9 meets two of the three criteria for a pandemic strain — it has the ability to infect people, and the human population does not have much immunity against the strain. What H7N9 can’t do — yet — is spread easily from person to person.